1. Field of the Invention
The present invention relates to a method for utilizing a methane-containing gas, which may be selected from the group consisting of pit gas, waste dump gas and biogas originating from fermentation installations and putrefication processes of sewage treatment plants, by feeding the methane-containing gas to a gas engine of a gas engine/generator assembly generating electricity.
2. Description of the Prior Art
It is known to install gas engine/generator assemblies in waste dumps and to operate the gas engine with waste dump gas. The waste dump gas sucked out of the waste has a content of about 50%, by volume, of methane. The remainder is comprised substantially of CO2 and some nitrogen. When the waste dump is closed, the production of biogas from the waste is slowly reduced over a period of about 10 years. The installed gas catching system then increasingly sucks air into the waste so that the methane content is reduced. When the methane content is reduced below 40%, by volume, it is impossible to operate the gas engine, and the waste dump gas must be burned off.
Pit gas is a gas consisting essentially of methane diluted by ventilation with air. The methane content of pit gas ranges from 30% to 50%, by volume, operating variations being unavoidable. When the methane content falss below 40%, by volume, gas engines fed by such a gas must be disconnected, and such a pit gas can no longer be used for generating electricity.
It is the primary object of this invention to make methane-containing gases, whose methane content is around or below 40%, by volume, useful as a fuel for gas engines to generate electricity.
According to the invention, this and other objects are accomplished in a method of the first-described type by passing combustion air containing an inert gas through a membrane separating installation to reduce the content of the inert gas before feeding the combustion air to the gas engine operated by the methane-containing gas.
By arranging a membrane separating installation downstream of the gas engine, the oxygen/nitrogen ratio in the combustion air fed to the gas engine is changed, i.e. the combustion air contains more oxygen and correspondingly less inert nitrogen. This reduction in the inert gas content of the combustion air compensates for the higher inert gas content in the methane-containing gas fed to the gas engine so that the gas engine may be operated under optimal conditions. This makes it possible to operate with so-called lean gases containing less than 40%, by volume, of methane an correspondingly higher contents of inert components, such as CO2 and/or nitrogen.
According to a preferred embodiment, the membrane separating installation comprises a gas permeation module with a membrane having a preferred permeability for oxygen, and condensed atmospheric air is fed to the gas permeation module to obtain a permeate whose inert gas content is reduced compared to that of the atmospheric air, and the permeate coming from the gas permeation module is fed as combustion air to the gas engine. Obviously, depending on the amount of combustion air and the desired reduction of nitrogen (inert gas), several gas permeation modules may be connected in parallel and/or in series.
The operation of the membrane separation installation may be so controlled that the oxygen/nitrogen ratio in the combustion air assures an optimal air ratio in the operation of the gas engine from a motor-technical and exhaust gas-technical point of view when the methane content of the methane-containing gas fed to the gas engine is less than 40%, by volume.
Preferably, the gas engine is operated with an air ratio xcex=1.5 to 1.8, most advantageously an air ratio xcex=1.6. Conventional gas engines may be operated at such an air ratio with a high engine efficiency and small exhaust gas emissions.